[stdlib] fix MutableSpan’s Sendable conformance

Author: glessard

stdlib/public/core/Span/MutableSpan.swift

@@ -57,7 +57,7 @@ public struct MutableSpan<Element: ~Copyable>
 
 @available(SwiftCompatibilitySpan 5.0, *)
 @_originallyDefinedIn(module: "Swift;CompatibilitySpan", SwiftCompatibilitySpan 6.2)
-extension MutableSpan: @unchecked Sendable where Element: Sendable {}
+extension MutableSpan: @unchecked Sendable where Element: Sendable & ~Copyable {}
 
 @available(SwiftCompatibilitySpan 5.0, *)
 @_originallyDefinedIn(module: "Swift;CompatibilitySpan", SwiftCompatibilitySpan 6.2)